(1) Field of the Invention
The present invention relates to an image processing apparatus for transporting a sheet subjected to image processing by a transport roller, a rotation control method for a motor provided in the image processing apparatus, and a computer-readable recording medium.
(2) Description of the Related Art
Examples of the image processing apparatus include an image forming apparatus, such as a printer, and the image forming apparatus includes a sheet transport device for transporting a sheet, such as paper, on which an image is to be formed.
In general, the sheet transport device includes transport rollers arranged along a sheet transport path at intervals shorter than the length of a sheet in a sheet transport direction, and sequentially transports the sheet from a transport roller located upstream in the sheet transport direction to a transport roller located downstream in the sheet transport direction.
As a specific example of transporting a sheet, a so-called registration operation is known in which a sheet is transported to a transfer position at the timing of forming an image on an image carrier.
The registration operation is performed as follows. Suppose that a pair of transport rollers is arranged along a transport direction, and that a pair of registration rollers is arranged more downstream than the pair of transport rollers. In this case, while a sheet is transported by the pair of transport rollers, a leading edge of the sheet in the transport direction is pressed against the pair of registration rollers which is in a state of non-rotation at the downstream side. When a loop (warp) is formed in a leading end portion of the sheet to correct skew (oblique transport), the rotation of the pair of transport rollers is temporarily stopped. Subsequently, the rotation of both the pair of transport rollers and the pair of registration rollers is restarted (i.e., the transport is resumed) at the timing of forming an image.
The temporary stop of the pair of transport rollers is performed by applying a brake to stop the rotation of a motor which is a drive source of the pair of transport rollers. The brake referred to here is, for example, a so-called short brake in which both ends of each coil of the motor are electrically shorted out. However, if, during a speed reduction period from the start of the brake application to the stop of the motor, the transport amount of a sheet is varied due to variations in the load on a drive mechanism for transmitting the drive force from the motor to the pair of transport rollers, then the size of the loop will vary. This may lead to a failure in skew correction.
A known method for suppressing occurrence of such variations is to perform a speed reduction control on the rotation of the motor.
Japanese Patent Application Publication No. 2004-85838 discloses a feedback control performed on a camera having a function of moving a lens (movable body) by a motor for the purpose of auto focus adjustment. In the feedback control adjustment, when the speed of the lens in motion is reduced to stop the lens, a difference between the actual speed of the lens and a target speed is determined based on the duration of each pulse interval of a pulse signal output along with the movement of the lens. If the actual speed is higher than the target speed, the brake is applied to the lens. If the actual speed is lower than the target speed, the speed of the lens is increased.
However, in the above case where the speed adjustment is performed based on the duration of each pulse interval while the speed of the motor is reduced, the following problem may occur. Specifically, in the above speed adjustment, the lower the speed of the motor, the longer the cycle of the pulse signal and the longer the pulse interval. As a result, the waiting time for pulse input becomes longer.
When the waiting time for pulse input becomes longer, even if there is an attempt to perform speed adjustment repeatedly during the time period from a low-rotation period, in which the motor is driven at low speed, to the stop of the motor, the number of times speed adjustment can be performed will be limited. This is because speed adjustment cannot be performed during the waiting time.
For example, suppose that the brake is applied in the first speed adjustment performed during the low-rotation period, and that the motor stops during the waiting time for pulse input for the second speed adjustment after the first speed adjustment. In this case, the brake of the first speed adjustment is continuously applied to the stop of the motor.
This means that even if the speed of the motor is slightly higher than the target speed at the time of the first speed adjustment, the brake is continuously applied after the first speed adjustment. As a result, the speed of the motor is reduced to a great extent, and the transport amount (stopping distance) traveled by a sheet during the period from the start of the speed reduction to the stop of the sheet becomes shorter than the original transport amount.
On the other hand, suppose that the speed of the motor is increased in the first speed adjustment, and that the motor stops during the waiting time for pulse input for the second speed adjustment after the first speed adjustment. In this case, even if the speed of the motor is slightly lower than the target speed at the time of the first speed adjustment, the speed of the motor is continuously increased. Since the speed of the motor is not reduced sufficiently, the stopping distance becomes longer than the original stopping distance. As a result, the time required from the start of speed reduction to the stop of the sheet (stopping time) becomes longer.
As described above, the speed adjustment control by increasing or decreasing the speed of the motor cannot stabilize the stopping distance during the period from the low-rotation period to the stop of the motor, and such a speed adjustment control may lengthen the stopping time.
When the stopping time of the motor is lengthened, and the resumption of the transport of the sheet during the registration operation is delayed, the sheet cannot be transported to the transfer position at an appropriate timing for image formation. To avoid such a situation, it is possible to delay the timing of image formation. However, this solution is not preferable since it will lower the productivity of image formation.
The above problem does not only apply to an image forming apparatus, but also to an image reading apparatus, such as a scanner, that transports a sheet (e.g., document) to a reading position to read an image on the sheet, and that has a function of temporarily stopping the transport of the sheet and resuming the transport of the sheet to the reading position at the timing of reading the image.
The above problem also applies to an image forming apparatus that does not need to form a loop to correct skew but has a function of: temporarily stopping a sheet that is being transported in a state where a leading edge of the sheet is in contact with a pair of registration rollers; and resuming the transport of the sheet at the timing of image formation.